SAN FRANCISCO — Federal investigators probing the cause of a massive Chevron refinery fire are focusing on possible corrosion in a decades-old pipe the company inspected late last year.

Investigators with the U.S. Chemical Safety Board said Saturday they are trying to determine why Chevron did not replace the pipe when it had a major five-year inspection last November.

That inspection led Chevron to replace an old pipe connected to the one that failed Monday, catching fire, sending up black smoke and causing thousands to seek medical attention for health issues in one of the most serious U.S. refinery fires in recent years.

Chevron said Saturday it too is seeking to understand why Monday’s accident occurred.

“We agree that this is a serious incident that warrants thorough investigation. We are cooperating with all regulatory agencies and are committed to better understanding the root cause of this incident,” said Sean Comey, a company spokesman.

The inspectors have not yet seen testing records for the pipe that failed, but given its age and the condition of pipes connected to it, they believe corrosion is a strong possible cause of its failure.

Also, investigators told The Associated Press that more than a dozen Chevron refinery workers were engulfed by vapor and narrowly escaped serious injury by under 30 seconds when the vapor ignited Monday.

“Witness testimony collected by CSB investigators indicates that a large number of workers were engulfed in the vapor cloud,” Dan Tillema, the board’s investigative team leader, said. “These workers might have been killed or severely injured, had they not escaped the cloud as the release rate escalated and the cloud ignited, shortly thereafter.”

The incident began Monday at 4:15 p.m., when a small dripping leak was detected by refinery workers. When engineers responded to find the leak’s cause, they removed insulation around the pipe. Shortly thereafter, a vapor cloud was released.

“Due to the high temperature of the material in the tower, in excess of 600 degrees Fahrenheit, the gas-oil immediately formed a large flammable vapor cloud,” the board’s investigators said. Interviews by investigators revealed that more than a dozen Chevron refinery workers were engulfed in the volatile vapor and escaped serious injury by fewer than 30 seconds when the diesel-like oil ignited.

County officials say sirens were activated to warn nearby residents of an accident, and the company alerted county officials later that a fire had ignited.

Chevron’s response — and Contra Costa County’s emergency warning system — is being investigated by the federal chemical safety agency and several other agencies.

Officials have said more than 4,000 people were treated and released at local hospitals in the days following the fire, and Chevron has set up a claims center for people who were affected. The claims center on Friday had already received 2,000 calls.

Investigators said, in general, all pipes corrode over time.

The pipe that failed Monday dated back to the 1970s, but it is still unclear if the thickness testing conducted by Chevron in its last major inspections noted corrosion in that specific, 8-inch pipe.

However, investigators said a 12-inch pipe connected to the one that sprung the leak Monday was found to be corroded, and was replaced after the November “turnaround,” an industry term for when a refinery unit is taken off-line so all the lines can be inspected.

Tillema said “important issues in the investigation included understanding why the pipe that later failed was kept in service during a late 2011 maintenance turnaround.”

By choosing not to replace the pipe that failed, he said Chevron had decided that it was strong enough to last another five years, when the next piping inspection would have been conducted, which is the industry standard.